This invention relates to a photographic light exposure meter and more particularly to a digital exposure meter for reading either incident or reflected illumination produced by high-intensity strobotron lights together with ambient light.
In photographing a subject illuminated by electronic flash lighting, i.e., strobotron lighting, it is normally difficult to accurately determine the correct exposure for a given camera shutter speed and film light sensitivity, i.e., film speed. Because of the extremely short duration of the strobotron flash, special exposure meters are necessary to make this measurement. As a result, numerous exposure readings are often required in doing portraiture work. It is therefore desirable to shorten the process by which the correct exposure is determined in order to minimize the inconvenience to the photographic subject. To the extent that the prior art exposure meters require synchronization by means of cables with the light sources or manual readjustments and calculations to determine the correct exposure for a given lighting setting, they complicate and slow the process and are thus undesirable. This is particularly true where the exposure meter must be readjusted for each new change in the lighting situation.
Another problem is that in order to get good depth of field by using a relatively small aperture opening, it is sometimes necessary to repeatedly fire the electronic flash to provide sufficient exposure. While some prior art digital exposure meters will accumulate the overall exposure and give a combined digital reading for the correct aperture, none of these exposure meters also displays the number of flashes required to produce the accumulated aperture setting. An even more serious disadvantage of some of these prior art exposure meters is that with either single or multiple flashes, the flashes must all take place within a relatively short period of time after the meter has been manually activated in order for the exposure meter to correctly read the flashes.
Still another problem of all prior art digital exposure meters of which the applicant is aware, is that the display is relatively small, due to the power limitations of a battery-operated exposure meter, so that the photographer must approach the subject closely in order to determine the correct reading. Also, the exposure meter must be reset manually after each flash reading, except in the multiple flash mode.
One of the more technical disadvantages of some prior art digital exposure meters lies in the circuit design. In this prior art circuit, the photo-optic sensor produces an electrical signal whose voltage is proportional to the intensity of the illumination. The voltage signal is amplified through an operational amplifier and is passed through a resistor to produce a current which is integrated by charging a capacitor. This prior art circuit design introduces a number of manufacturing problems. One problem is that operational amplifiers which do not introduce a voltage off-set, a necessary condition of the circuit design, are extremely expensive. Another problem is that they introduce quality control problems in manufacturing. Still another problem is that with electronic flash equipment in which the duration of the flash is automatically controlled by the flash unit, the intensity of the flash is extremely high and the duration may vary between 1/500 of a second and 1/30,000 of a second. With such high-intensity equipment, many prior art photo-optic sensor systems saturate and do not fully integrate the sensor signal during the first part of the flash, thereby producing inaccurate readings.